shipping problem: Attribute mismatch: AMOUNT. Order number 318

I am facing the problem, that if I am purchasing exactly 3(!) obejcts of one product which is 1.0000kg heavy, get the error when paying with saferpay over the addon Saferpay: "Attribute mismatch: AMOUNT. Order number 318"
This is strange and in my opinion, it has something to do with the shippping calculation I am doing with the addon: Calculate for price/postal code
See the image to understand, how the delviveryprice is calculated.
Now, I hope this is giving you a clue:
If I set the deliveryprice to 0.0 Fr, it works fine, but If i set it to 7Fr, I get the error.

So I am puzzled with this error and dont know how to solve it.
thanks for your help